This project is a continuation of a portion of the project entitled The Molecular Pathogenesis of Multiple Myeloma, Z01 SC 006581. Progress during the past year has resulted in several publications, and added to previous accomplishments. First, comprehensive FISH analyses on metaphase chromosomes in 47 multiple myeloma cell lines and 48 advanced multiple myeloma tumors showed the following: 1) primary IgH translocations, mostly balanced, involving 5 recurrent partners were present in 68% of cell lines, 70% of non-hyperdiploid tumors, but only 12% of hyperdiploid tumors;2) secondary translocations (IgH translocations not involving the five recurrent loci, IgL and IgK translocations, and MYC translocations) usually are unbalanced, with complex structures, and occur with a similar prevalence in hyperdiploid and non-hyperdiploid tumors;3) two of the 5 recurrent translocations rarely occur within the same tumor cell;and 4) the 5 recurrent translocations sometimes can be secondary events. Second, we summarized FISH and array comparative genomic hybridization analyses on 47 multiple myeloma cells lines that enabled us to detect and characterize MYC rearrangements in 43 (&gt;90%) of the cell lines. Together with our previous results, this gave us further support for our hypothesis that MYC rearrangements represent a late progression event that provides a general paradigm for secondary translocations in multiple myeloma. Third, we published two papers in which we cloned and/or characterized by FISH translocation breakpoints that are found in complex translocations that involve MYC. Fourth, we have cloned from two MM cell MYC translocations in which no Ig sequences are involved, but one of which involves BLIMP, a master regulator of normal plasma cell differentiation. Fifth, we have compared array CGH data on 50 MM cell lines with array CGH data on 155 MM tumors that was made publically available by the MMRC. These results show that MYC rearrangements are much more frequent in primary MM tumors than found by others with FISH analyses, i.e., about 32% of newly diagnosed MM tumors and 46% of previously treated tumors. Sixth, we analyzed the MMRC data set that included gene expression profiling and CGH arrays for 155 tumors to show that MYC expression is significantly increased in tumors that have rearrangements in the MYC locus. Finally, were were able to identify polymorphisms in MYC in two of the four MM cell lines that have no detectable rearrangement by either FISH or array CGH analyses;both informative lines express both MYC alleles to a similar extent. This contrasts to previous results on thirteen MM cell lines that have MYC rearrangements and MYC polymorphisms, for which we showed selective expression of only one MYC allele. So, for the first time we can concludes that MYC expression in MM cell lines need not be associated with rearrangements or other structural changes in the MYC locus that dysregulate the expression of only one MYC gene.